1. Field of the Invention
The present invention relates to a diagnosis device of an exhaust purification catalyst such as a NOx occlusion reduction catalyst.
2. Description of Related Art
In recent years, a device using a NOx occlusion reduction catalyst, i.e., a NOx catalyst, has been gathering attention as one of exhaust purification devices for purifying NOx (nitrogen oxides) contained in exhaust gas. The NOx catalyst used in the device consists of an alkaline earth material (occlusion material) and platinum, for example. The NOx catalyst has characteristics of occluding the NOx in the exhaust gas when the atmosphere of the exhaust gas is a lean air fuel ratio (i.e., air fuel ratio corresponding to fuel ratio lower than that of stoichiometric air fuel ratio) and of reducing and removing the occluded NOx with reduction components such as HC and CO contained in the exhaust gas when the air fuel ratio is rich (i.e., air fuel ratio corresponding to fuel ratio higher than that of stoichiometric air fuel ratio). This device uses such the characteristics of the catalyst. The device purifies the NOx contained in the exhaust gas and reduces the NOx emission quantity by repeating the occlusion and the reduction (discharge) of the NOx with the catalyst.
However, also in such the device, there is limitation in the occlusion capacity of the NOx catalyst. Therefore, if the catalyst is continuously used in the environment where the NOx reduction quantity (NOx discharge quantity) exceeds the NOx occlusion quantity and the NOx occlusion quantity approaches the occlusion limitation, the NOx purifying performance of the catalyst falls significantly. Therefore, conventionally, as processing (catalyst recovery processing) for recovering from the lowering of the NOx purifying performance (temporary performance degradation corresponding to NOx occlusion quantity), reduction removal of the NOx occluded in the NOx catalyst is performed periodically. For example, a diagnosis device of an exhaust purification catalyst that diagnoses a degree of performance degradation (for example, existence or nonexistence of sulfur poisoning) of a NOx catalyst resulting from a sulfur component (S) and the like contained in engine fuel and the like during catalyst recovery processing is also known, for example, as described in Japanese Patent Gazette No. 2692380. Next, with reference to FIG. 9, an outline of the catalyst diagnosis processing currently performed by conventional and general devices including the above-described device will be explained. In order to perform recovery and diagnosis of the NOx catalyst provided in an engine exhaust system, the device provides a LNT (Lean NOx Trap) system that makes an air fuel ratio rich temporarily to reduce the occluded NOx by performing oversupply of the fuel for combustion (i.e., by performing rich purge). Here, an example in which the device is applied to an exhaust purification system for a vehicular diesel engine that performs steady operation by lean combustion will be explained.
FIG. 9 is a timing chart showing whether the rich purge is performed and the air fuel ratios A/F (equivalent to oxygen concentrations in exhaust gas) upstream and downstream of the NOx catalyst with respect to a flow direction of the exhaust gas. The upstream air fuel ratio A/F (oxygen concentration) and the downstream air fuel ratio A/F (oxygen concentration) can be sensed respectively, e.g., with A/F sensors (oxygen concentration sensors) provided upstream and downstream of the NOx catalyst with respect to the flow direction of the exhaust gas.
The device performs the rich purge as the catalyst recovery processing periodically (i.e., at each elapse of predetermined time). That is, for example, as shown in FIG. 9, the oversupply of the fuel is performed to the engine to start the rich purge at timing t51. Then, as shown in FIG. 9, the air fuel ratio A/F upstream of the NOx catalyst with respect to the flow direction of the exhaust gas becomes rich temporarily. Thus, the occluded NOx is reduced and removed with HC, CO and the like contained in the exhaust gas, so the occluded NOx is discharged from the catalyst. As shown by a solid line L51a in FIG. 9, the air fuel ratio A/F downstream of the NOx catalyst with respect to the exhaust gas flow is maintained at the stoichiometric air fuel ratio A/F0 during reduction removal (discharge) of the occluded NOx.
In the example shown in FIG. 9, the reduction removal (discharge) of the occluded NOx ends at timing t52. The end timing can be detected based on a change in the air fuel ratio A/F downstream of the NOx catalyst with respect to the flow direction of the exhaust gas. In detail, when the air fuel ratio A/F shifts from the stoichiometric air fuel ratio A/F0 to the rich side and falls below a predetermined end determination value (threshold TH1) to the richer side, it is determined that the discharge of the occluded NOx ends and the rich purge is ended (stopped) as shown in FIG. 9. Thus, the air fuel ratio A/F returns to the lean state of the steady operation. The threshold TH1 is set slightly on the richer side than the stoichiometric air fuel ratio A/F0. If the rich purge is continued though the discharge of the occluded NOx ends, there is s possibility that the emission is deteriorated by the discharge of the unburned fuel (i.e., rich exhaust gas). The threshold TH1 is set so that the rich purge can be stopped (ended) immediately after the end of the discharge of the occluded NOx is determined. However, with such the setting, as shown by a broken line L51b in FIG. 9, the occluded NOx cannot be discharged thoroughly, and a certain quantity of the occluded NOx remains as shown by an area R1 in FIG. 9 (area R1 correlates with remaining quantity).
The device periodically performs the rich purge as the catalyst recovery processing to reduce and remove the substantially entirety of the occluded NOx. Thus, the device periodically recovers the purifying performance (exhaust purification performance) of the NOx catalyst.
The device diagnoses the degree of the performance degradation of the catalyst based on the NOx quantity occluded by the catalyst per unit time, i.e., the NOx quantity (NOx occlusion quantity per unit time) occluded from the time when the previous rich purge is performed to the time immediately before the present rich purge is performed. When the NOx occlusion quantity is small, it is determined that the degree of the performance degradation of the catalyst is high and suitable recovery processing is performed, for example. The NOx catalyst is set under a purifying condition (environment enabling discharge of occluded NOx) only while the rich purge is performed. Therefore, the NOx occlusion quantity per unit time (NOx occlusion quantity before starting rich purge) can be estimated based on a time length (unit time t51-t52) from the timing t51, at which the rich purge is started, to the timing t52, at which the reduction removal (discharge) of the occluded NOx ends. It can be determined that the NOx occlusion quantity increases as the time length t51-t52 lengthens. At this time, a certain estimation error arises due to the residual NOx approximately corresponding to the area R1 in FIG. 9. Therefore, it is desirable to correct the estimation error by suitable computation or the like.
Thus, the catalyst can be recovered periodically even with the conventional device including the device described in Japanese Patent Gazette No. 2692380. The device can detect the degradation of the catalyst in an early stage and perform the recovery processing concerning the sulfur poisoning and the like by performing the degradation diagnosis when performing the recovery processing. However, the purifying performance of the catalyst is significantly affected by environmental temperature. Usually, the purifying performance of the catalyst declines and the reduction speed of the NOx slows down if the temperature of the catalyst is higher or lower than appropriate temperature. Therefore, when the catalyst recovery processing and the catalyst diagnosis processing are performed under such the temperature environment, the air fuel ratio A/F downstream of the NOx catalyst with respect to the exhaust gas flow direction falls below the predetermined end determination (threshold TH1) to the richer side and the rich purge is ended (stopped) in an early stage (for example, at timing t53) where a residual NOx quantity is still large as shown by a chain double-dashed line L52a in FIG. 9. Due to the large residual NOx substantially shown by an area R2 in FIG. 9 (area R2 corresponds to remaining quantity), the estimation accuracy of the NOx occlusion quantity (NOx occlusion quantity before starting rich purge) is deteriorated. If such the large error arises, thorough correction cannot be performed even if the suitable correction calculation is performed, for example, in accordance with the catalyst temperature. Accordingly, the deterioration of the diagnosis accuracy of the catalyst diagnosis processing is unavoidable.
In order to inhibit such the deterioration of the diagnosis accuracy, a scheme of altering the threshold TH1 to the richer side (for example, to threshold TH2 in FIG. 9) to reduce the residual NOx quantity as of the end of the rich purge may be employed. However, in this case, there is a concern about the deterioration of the emission as mentioned above. Therefore, it is difficult for the device described in Japanese Patent Gazette NO. 2692380 to perform the catalyst diagnosis processing with high reliability and accuracy while maintaining good emission.
There is another device that performs the catalyst diagnosis processing only when the catalyst temperature is within a predetermined range. However, there is a concern that the device cannot perform the catalyst diagnosis processing over a long period of time depending on the temperature environment of the catalyst, so it is difficult to detect the degradation of the catalyst in an early stage with high reliability.